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1 Introduction Draft 9th Brochure 16 December 2013 1 Introduction 1.1 Quantities and units The value of a quantity is generally expressed as the product of a number and a unit. The unit is simply a particular example of the quantity concerned which is used as a reference, and the number is the ratio of the value of the quantity to the unit. For a particular quantity many different units may be used. For example the speed v of a particle may be expressed in the form v = 25 m/s = 90 km/h, where metre per second and kilometre per hour are alternative units for expressing the same value of the quantity speed. However, because of the importance of a set of well defined and easily accessible units universally agreed for the multitude of measurements that support today’s complex society, units should be chosen and defined so that they are readily available to all, are constant throughout time and space, and are easy to realise with high accuracy. When an experimental measurement of a quantity is reported, two results are required: the estimated value of the measurand (the quantity being measured), and the estimated uncertainty of that value. Both are expressed in the same unit. The uncertainty is a measure of the accuracy of the measured value, in the sense that a lower uncertainty corresponds to a more accurate and more precise measurement. A simple measure of the uncertainty in a measurement result may sometimes be provided by the width of the probability distribution of repeated measurements. In order to establish a system of units, such as the International System of units, the SI, it is necessary first to establish a system of quantities, including a set of For example the quantity speed, v, may be equations defining the relations between the quantities. This is necessary expressed in terms of because the equations between the quantities determine the equations relating distance x and time t by the units, as described below. Thus the establishment of a system of units, the equation v = dx/dt. If the metre m and second s which is the subject of this brochure, is intimately connected with the algebraic are used for distance and equations relating the corresponding quantities. time, then the unit used for speed v might be metre per second, m/s. As new fields of science develop, new quantities are devised by researchers to represent the interests of the fields. With these new quantities come new As a further example, in equations relating them to the quantities that were previously familiar, and these electrochemistry the electric mobility of an ion new relations allow us to establish units for the new quantities that are related to u is defined as the ratio of the units previously established. In this way the units to be used with the new its velocity v to the quantities may always be defined as products of powers of the previously electric field strength E: u = v/E. The unit of established units. electric mobility is then given as (m/s)/(V/m) = 2 −1 −1 The definition of the units is established in terms of a set of defining constants, m V s , where the volt per metre V/m is used for which are chosen from the fundamental constants of physics, taken in the the quantity E. Thus the broadest sense, which are used as reference constants to define the units. In the relation between the units SI there are seven such defining constants. From the units of these defining is built on the underlying relation between the constants the complete system of units may then be constructed. These seven quantities. defining constants are the most fundamental feature of the definition of the entire system of units. 1/29 Draft 9th Brochure 16 December 2013 Historically the units have always previously been presented in terms of a set of seven base units, all other units then being constructed as products of powers of the base units which are described as derived units. The choice of the base units was never unique, but grew historically and became familiar to users of the SI. This description in terms of base and derived units remains valid, although the seven defining constants provide a more fundamental definition of the SI. It is tempting to think that there is a one-to-one correspondence between the base units and the defining constants, but that is an oversimplification which is not strictly true. However these two approaches to defining the SI are fully consistent with each other. 1.2 The International System of units, SI, and the corresponding system of quantities This brochure is concerned with presenting the information necessary to define and use the International System of Units, universally known as the SI (from the French Système International d’Unités). The SI was established by and is defined by the General Conference on Weights and Measures, CGPM, as described in section 1.8 below. The system of quantities used with the SI, including the equations relating the quantities, is just the set of quantities and equations that are familiar to all scientists, technologists, and engineers. They are listed in many textbooks and in many references, but any such list can only be a selection of the possible quantities and equations, which is without limit. Many of the quantities, with their corresponding names and symbols, and the equations relating them, were listed in the international standards ISO 31 and IEC 60027 produced by Technical Committee 12 of the International Organization for Standardization, ISO/TC 12, and by Technical Committee 25 of the International Electrotechnical Commission, IEC/TC 25. These standards have been revised by the two organizations in collaboration, and are known as the ISO/IEC 80000 Standards, Quantities and Units, in which the corresponding quantities and equations are described as the International System of Quantities.. The base quantities used in the SI are time, length, mass, electric current, thermodynamic temperature, amount of substance, and luminous intensity. The corresponding base units of the SI were chosen by the CGPM to be the second, metre, kilogram, ampere, kelvin, mole, and candela. The history of the development of the SI is summarized in section 1.8 below. Acronyms used in this brochure are listed with their meaning on p. XX. In these equations the electric constant 0 (the permittivity of vacuum) and the magnetic 2 constant µ0 (the permeability of vacuum) have dimensions and values such that 0µ0 = 1/c , where c is the speed of light in vacuum. Note that the electromagnetic equations in the CGS-EMU, CGS-ESU and Gaussian systems are based on a different set of quantities and equations in which the magnetic constant µ0 and the electric constant ε0 have different dimensions, and may be dimensionless. 2/29 Draft 9th Brochure 16 December 2013 1.3 Dimensions of quantities By convention physical quantities are organised in a system of dimensions. Each of the seven base quantities used in the SI is regarded as having its own dimension, which is symbolically represented by a single roman capital letter. The symbols used for the base quantities, and the symbols used to denote their dimension, are as follows. Table 1. Base quantities and dimensions used in the SI ____________________________________________________________ Quantity symbols are always written in an italic Base quantity Symbol for quantity Symbol for dimension font, symbols for units in a roman (upright) font, and ____________________________________________________________ symbols for dimensions in sans-serif roman capitals. length l, x, r, etc. L For some quantities a mass m M variety of alternative time, duration t T electric current I, i I symbols may be used (as thermodynamic temperature T Θ for length and electric amount of substance n N current in the table). luminous intensity I J v Symbols for quantities are ____________________________________________________________ recommendations, in contrast to symbols for units (which appear elsewhere in All other quantities are derived quantities, which may be written in terms of this Brochure) which are base quantities by the equations of physics. The dimensions of the derived mandatory, and quantities are written as products of powers of the dimensions of the base independent of the quantities using the equations that relate the derived quantities to the base language. quantities. In general the dimension of any quantity Q is written in the form of a dimensional product, dim Q = Lα Mβ Tγ Iδ Θε Nζ Jη where the exponents α, β, γ, δ, ε, ζ, and η, which are generally small integers which can be positive, negative, or zero, are called the dimensional exponents. The dimension of a derived quantity provides the same information about the relation of that quantity to the base quantities as is provided by the SI unit of the derived quantity as a product of powers of the SI base units. There are some derived quantities Q for which the defining equation is such that all of the dimensional exponents in the equation for the dimension of Q are zero. This is true in particular for any quantity that is defined as the ratio of two quantities of the same kind. Such quantities are described as being dimensionless, and are simply numbers. However the coherent derived unit for such dimensionless quantities is always the number one, 1, since it is the ratio of two identical units for two quantities of the same kind. For that reason dimensionless quantities are sometimes described as being of dimension one. There are also some quantities that cannot be described in terms of the seven base quantities of the SI at all, but have the nature of a count.
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